Filament coating composition



United 1* This invention relates to new and improved compositions useful for the coating of wire 'fila'rnent's. More particularly, the invention relates to coating" compositions useful in the preparation of coated wire filaments for use in electron-emitting devices.

Heretofore, coated wire filaments for use in electronemitting devices such as lamps, etc., generally have been produced by coating a wire filament such as a tungsten filament, with an alkaline earth metal carbonate mixture. This mixture is generally composed of an alkaline earth metal carbonate powder suspended in an organic liquid containing a binder in a solvent, e.g., a nitrocellulose lacquer dissolved in butyl acetate. After the coating is applied, it is converted to a hard durable coating by sintering, that is, converting the alkaline earth met-a1 carbonate to an oxide by heating to an elevated temperature.

Such coating compositions, however, are not completely satisfactory. For example, only a thin coating can be applied to the filamentssince the proportion of solids in the coating compositions is relatively low. Moreover, when applying the coating composition, it is difficult to obtain a coating of uniform thickness and, also, it is difficult to keep the composition from flowing over a greater portion of the filament than is desired. Even after the coating of the filament has been performed satisfactorily, accidental exposure to heat for even a few seconds may bake the lacquer destroying the bond between the coating and the wire. Additional difliculties are also encountered in the conversion of the coating to the oxide. For example, it is diflicult to get rid of the binder employed to hold together the fine carbonate particles during the mechanical processing steps in the completion of the lamp. In many cases, the binder, instead of being vaporized off during the sintering operation, is merely converted into a carbonaceous deposit. To insure complete removal of the binder, it is necessary to supply extra amounts of air to furnish oxygen necessary to convert the binder to a gaseous form. Furthermore, even if the binder is successfully vaporized, voids remain in the sintered coating at the point where the binder is removed. These porous coatings are quite fragile and this in turn necessitates greater care in the handling of the filaments during the subsequent mechanical processing to prevent flaking off of the coating and ultimate rejection of lamps containing such defective filaments. Even if the filament is successfully incorporated-into a lamp, the service life thereof is reduced due to the relatively small amount of coating.

Through the years, attempts have been made to overcome the above difficulties. McCullochPatent No. 1,939,-

075 describes a method in which water and an acid are 2 patent, the coatings produced from such suspensions are much heavier than those made using solvents. 7

However, the development of coiled filaments made successful coating much more diificult. For example, for a lamp to have a reasonably long service life, it is necessary that the surface of the coil be completely and uniformly coated with a heavy coating. Coating compositions of a thick consistency such as those described in the McCulloch patent, newe /a ain not produce a heavy uniform coating butratlrer coatedonly the outer portions of the coil with a relatively heavy coating, while the inside portions had very little if any coating thereon. As a result, when coating coiled filaments, it is necessary to employ thinnercoating compositions such as the compositions described above containing the carbonate in an organic liquid. However, even though these coating compositions are superior to the thick compositions, the difliculties described above still exist.

In accordance with the present invention, it has been found that new andimproved coating compositions can be prepared which produce heavy, dense, uniform, tightly adherent coatings on coiled filaments without employing any binders or solvents. Moreover, the coating compositions of the present invention are of a sufficiently thin consistency that they can be easily applied to completely coat the entire surface of the coils and require only one coat to achieve a tightly adherent, uniform, heavy coating on the filament. Also, filaments coated in accordance with the method of the present invention can be easily handled during mechanical processing with a minimum of care since the coating thereon is much more adherent and durable than previous coatings. Furthermore, the coating compositions o'ftlie present invention, although of a thin consistency when applied to the filament,- almost immediately become immobile. Thus, no drying of the coated filaments with heat is necessary before resumption of the mechanical processing steps in the completion of the lamps. In addition, the absence of any binder eliminates the possibility of the coating flaking off due to decomposition of the binder.

The coating composition of the present invention coinprises a suspension of at least one alkaline earth metal carbonate in a dilute aqueous solution of an alkaline earth metal salt of a short chain aliphatic carboxylic acid, the carboxylic acid salt concentration comprising between about 0.35 and 2.0 mol percent of the alkaline earth metal carbonate. The alkaline earth metal carbonate or carbonates are preferably employed in the form of finely divided powders (4 to 10 microns) since the fine powders result in a more dense coating on the filament.

Particularly useful alkaline earth metal carbonates for preparing the coating compositions of the present invention are the carbonates of barium, calcium, and strontium. Advantageously, mixtures of the three carbonates are employed and, especially advantageously, mixed carbonates salts are those containing the alkaline earth metals specified above, and the preferred salts are those of barium, calcium, and strontium. The short chain aliphatic carboxylic acids are preferably those acids containingbetween about 2 and 6 carbon atoms such as, for

' example, acetic, propionic, butyric, valeric acids, isomers thereof, and the like. Particularly useful acids are acetic and propionic acids.

The coating compositions of the present invention may be prepared by combining the alkaline earth metal carbonate powder with water and one of the aliphatic carboxylic acids above. When these ingredients are combined, the acid reacts with the alkaline earth metal carbonate to produce the required salt. Alternatively, the coating compositions may be prepared by combining the previously prepared salt with the carbonate and the water. However, in any case, the proportion of the aliphatic carboxylic acid salt present in the coating composition should be between about 0.35 and 2.0 mol percent of the alkaline earth metal carbonate therein and preferably between 0.75 and 1.5 mol percent. The use of less than about 0.35 mol percent or more than about 2.0 mol percent causes the consistency of the composition to thicken to the point that it does not coat the inside portions of the coiled filament satisfactorily.

The quantity of water in the coating composition of the present invention is preferably kept at a minimum so that a heavier coating can be obtained for a given volume of liquid applied. In preparing the coating compositions, it has been found that only a small quantity of water is required to obtain a fluid composition of a thin consistency similar to thin paint. In general, the amount of water used will be less than about 50% by weight of the composition. Usually only about to 35% is required and for most purposes only about to 30% is preferred.

The fact that the coating compositions of the present invention are liquids of a thin consistency even when only a relatively small amount of water is used results in the achievement of several heretofore unobtainable objectives. For example, a heavy, dense, uniform coating can be obtained on a coiled filament in only one coating step. Moreover, even though the composition has a thin consistency, once the coating is placed on the filament, there is substantially no running of the coating composition beyond the portion of the filament being coated. The evaporation of only a small quantity of, water renders the coating immobile. This change in form is believed to be the result of the thixotropic character of the coating composition.

Although the mechanism by which the coating compositions of the present invention achieve the superior results discussed above is not actually known, it is theorized that the presence of the very small amount of the ionized alkaline earth metal carboxylic acid salt creates a charge on each individual particle of the carbonate powder. This charge causes the particles to repel each other and this, in turn, prevents them from clinging or adhering to one another and settling out of the suspension. The small amount of water present in the coating composition creates a thin film between the individual particles and serves as a lubricating agent for the particles to give the composition its apparent thin consistency.

The coating compositions of the present invention are particularly useful in the production of coated coiled filament for use in fluorescent lamps. To obtain a long service life with fluorescent lamps, it is necessary that the coiled filament have a dense heavy coating thereon. since during the operation of the lamp, the activated coating is gradually vaporized. When the coating is depleted, the lamp fails. Also, it is necessary that the atmosphere in a fluorescent lamp contain no impurities such as result when small quantities of binder vaporize after the lamp is in use.

The coating compositions of the present invention provide superior results in fluorescent lamps since with a single coating step it is possible to obtain a heavy, dense, uniform, tightly adherent coating on the filament. This results in extra long lamp life. In addition, the absence of any binders or solvents reduces the possibility of impurities in the atmosphere of the lamp, thus also assuring a long lamp life.

I The invention will be more fully described by the following examples. However, it is not intended that the invention be limited by such examples.

Example I added to prevent premature darkening of the ends of fluorescent lamps.

' earth metal oxides.

The resulting mixture was carefully mixed and then milled in a pebble mill for several hours. The resultant suspension was stable and had a consistency similar to thin paint. The apparent specific gravity of the suspension was 2.3.

A portion of the above suspension was applied to coiled tungsten filaments by dipping the filaments into the suspension and blowing off the excess. The coiled filaments had a uniform, tightly adherent coating over both the inside and outside portions of the coils. The filaments were then air dried for several hours and assembled into a cathode assembly. Two assemblies prepared as above were sealed into the ends of a fluorescent lamp tube. The filaments were then heated to convert the carbonate coating to the respective alkaline The lamp was vacuum exhausted and flushed with argon several times to remove any extraneous gas, and then filled with argon and a small amount of mercury before being sealed.

A number of fluorescent lamps containing filaments coated with the compositions of the present invention were subjected to standard lamp life .tests and it was found that none of the samples failed even after 4000 hours. In comparative tests with conventionally produced fluorescent lamps containing filaments coated with compositions containing organic liquids, a number of lamps failed in less than 3000 hours.

Example II The procedure of this example was the same as Example I except that in place of the acetic acid an equal. amount of propionic acid was employed. The resulting coating composition and filaments coated therewith proved to be equally as good as the compositions and filaments produced in Example I.

In the same manner, butyric acid and valeric acid can be substituted for the acetic acid of Example I to achieve similar results.

Example III The procedure of this example was the same asv Example I except that 32 grams of barium acetate were sub stituted for the acetic acid. The resulting suspension appeared to be identical in every respect to the suspensions. prepared in Examples I and II and lamps made with' filaments coated with this suspension also achieved the superior results shown by the lamps prepared according to the procedure of the previous examples.

The following examples illustrate the use of greater amounts of acid and the use of acids other than the acids and amounts thereof employed in the coating compositions of the present invention.

Example IV hours. The milling was then continued for an additional 24 hours, at the end of which period the suspension had a consistency similar to extremely heavy whipped cream. The apparent density was about 1.8.

A portion of the above suspension was applied to coiled tungsten filaments. The suspension did not penetrate the inside of the turns of the coils but merely clogged the outside of the coils. No satisfactory coatings could be obtained. The suspension also had a marked tendency to settle. At this point, further testing of the suspension was abandoned.

A comparison of these failures with the superior results obtained in Examples I, II, and III clearly shows that the compositions of the present invention are completely different from the compositions of this example. Whereas the coating compositions of the present invention contained a higher proportion of solids, the suspension had a much thinner consistency and could be easily applied to coiled tungsten filaments to produce heavy, dense, uniform, tightly adherent coatings. Moreover, the use of such coated filaments in fluorescent lamps as shown above produced results much superior to those achieved with conventional fluorescent lamps. In contrast, no satisfactorily coated filaments could even be made using the suspension of this example.

Example V To a solution of 21 grams of nitric acid in 1500 grams of water were added 2000 grams of a triple carbonate similar to that employed in the previous examples. The resulting mixture was a thick moist paste and remained in this condition even after milling for 24 hours. No filaments were coated with the above composition since it was obvious that satisfactory coatings would be impossible.

From the above examples, it is apparent that only certain acids and salts thereof can be employed in the coating compositions of the present invention to achieve the superior results described above. Also, the examples clearly show that not only are the acids and salts restricted to the short chain aliphatic carboxylic acids and alkaline earth metal salts thereof but, also, the amount of the acid or salt must be controlled within a certain definite range.

When the particular acids are employed in the particular amounts set forth above, coating compositions can be prepared which produce in a single coating step a heavy, dense, uniform, tightly adherent coating on a coiled filament for use in an electron-emitting device. Moreover, the coating compositions of the present invention can be easily applied to produce these superior coatings since the compositions are of a relatively thinconsistency, and even though the compositions have such a thin consistency, almost immediately upon being applied to the filament, they become immobile and do not run beyond the portion being coated. Furthermore, since the coating solidifies so quickly, no drying of the coated filaments with heat is necessary before the assembly of the filament into the lamp.

Although the foregoing discussion has described in detail the coating of filaments for lamps, the present invention is applicable also to the coating of other electronemitting elements where it is desirable to have a suitable dense adherent coating. Thus, it is to be understood that the expression filament is intended to include such other electron-emitting elements.

Furthermore, from the above description, it will be apparent to one skilled in the art that various modifications can be made within the scope of the invention. Therefore, the invention is to be limited only by the following claims.

What is claimed is:

1. A composition suitable for the coating of filaments which consists essentially of at least one alkaline earth metal carbonate suspended in an aqueous solution of an alkaline earth metal salt of an aliphatic carboxylic acid having between 2 and 6 carbon atoms, and said carboxylic acid salt comprising between about 0.35 and 2.0 mol percent of said alkaline earth metal carbonate; the proportion of water being less than about 50% by weight of said composition.

2. A composition suitable for the coating of filaments which consists essentially of at least one alkaline earth metal carbonate suspended in an aqueous solution of an alkaline earth metal salt of an aliphatic carboxylic acid having between 2 and 6 carbon atoms, said alkaline earth metals being selected from the group consisting of barium, calcium, and strontium, and said carboxylic acid salt comprising between about 0.35 and 2.0 mol percent of said alkaline earth metal carbonate; the proportion of water being less than about 50% by weight of said composition.

3. A composition suitable for the coating of filaments which consists essentially of a mixture of barium carbonate, calcium carbonate, and strontium carbonate, suspended in an aqueous solution of an alkaline earth metal salt of an aliphatic carboxylic acid having between 2 and 6 carbon atoms, said alkaline earth metal being selected from the group consisting of barium, calcium, and strontium, and said carboxylic acid salt comprising between about 0.35 and 2.0 mol percent of said alkaline earth metal carbonates; the proportion of water being less than about 50% by weight of said composition.

4. A composition suitable for the coating of filaments which consists essentially of a finely divided co-precipitated mixture of barium carbonate, calcium carbonate, and strontium carbonate, suspended in an aqueous solution of an alkaline earth metal salt of aliphatic carboxylic acids having from 2 to 6 carbon atoms, said alkaline earth metal being selected from the group consisting of barium, calcium, and strontium, and said carboxylic acid salt comprising between about 0.35 and 2.0 mol percent of said alkaline earth metal carbonates; the proportion of water being less than about 50% by weight of said composition.

5. A composition suitable for the coating of filaments which consists essentially of a finely divided co-precipitated mixture of barium carbonate, calcium carbonate, and strontium carbonate, suspended in an aqueous solution of an alkaline earth metal salt of aliphatic carboxylic acids having from 2 to 6 carbon atoms, said alkaline earth metal being selected from the group consisting of barium, calcium, and strontium, and said carboxylic acid salt comprising between about 0.75 and 1.5 mol percent of said alkaline earth metal carbonates; the proportion of water being less than about 50% by weight of said composition.

6. A composition suitable for the coating of filaments which consists essentially of a finely divided co-precipitated mixture of barium carbonate, calcium carbonate, and strontium carbonate, suspended in an aqueous solution of an alkaline earth metal acetate selected from the group consisting of barium acetate, calcium acetate, and strontium acetate, and said acetate comprising between about 0.35 and 2.0 mol percent of said alkaline earth metal carbonates; the proportion of water being less than about 50% by weight of said composition.

References Cited in the file of this patent UNITED STATES PATENTS 1,894,059 Rufiiey Jan. 10, 1933 1,939,075 McCulloch Dec. 12, 1933 2,346,553 Brown Apr. 11, 1944 2,535,999 Bouchard Dec. '26, 1950 2,547,869 Keilholtz Apr. 3, 1951 2,559,530 Becker July 3, 1951 2,673,387 Forker Mar. 30, 1953 2,724,070 Heine et a1. Nov. 15, 1955 2,830,027 Schweitzer Apr. 8, 1958 2,831,137 Roebling Apr. 15, 1958 

1. A COMPOSITION SUITABLE FOR THE COATING OF FILAMENTS WHICH CONSISTS ESSENTIALLY OF AT LEAST ONE ALKALINE EARTH METAL CARBONATE SUSPENDED IN AN AQUEOUS SOLUTION OF AN ALKALINE EARTH METAL SALT OF AN ALIPHATIC CARBOXYLIC ACID HAVING BETWEEN 2 AND 6 CARBON ATOMS, AND SAID CARBOXYLIC ACID SALT COMPRISING BETWEEN ABOUT 0.35 AND 2.0 MOL PERCENT OF SAID ALKALINE EARTH METAL CARBONATE, THE PROPORTION OF WATER BEING LESS THAN ABOUT 50% BY WEIGHT OF SAID COMPOSITION. 